A “powder” or “dry” aerosol, as used herein, is a relatively stable suspension of fine solid particles in a gas, especially air. There is an increasing need for reliable data on the effects of short term exposure to many toxic chemical, biological and radiological powder aerosols. For example, inhalation toxicology studies are needed to determine the effects of brief exposures to atmospheres containing minute amounts of highly toxic chemical, biological, or radiological threat agents. Such studies present numerous technical challenges and, thus, there is a shortage of useful data on the effects of short term inhalation exposure to many toxic powder aerosols.
It is difficult to achieve a controlled mass flow rate of a powder aerosol. Gravitational, fluid dynamic, and interactive forces, such as electrostatic, van der Waals, and capillary forces, can introduce significant flow rate variations. Electrostatic charge interactions between dry aerosol particles and device surfaces can also disrupt aerosol flows. Additionally, particle size distribution, particle morphology, and moisture content can influence powder aerosol flow rates.
As the toxicity of the test substances increases, exposure periods and concentrations must be controlled with greater precision. Short duration tests of extremely toxic dry aerosols can range from several seconds to 30 minutes or more, and aerosols must often be generated from very small quantities of powder, typically in the range of 25-500 mg. Particles may be less than 1 micron in diameter, can become easily airborne, disseminate rapidly in the air and adhere to a variety of surfaces. Precautions taken in handling and containment of test substances greatly complicate test administration. Simple procedures such as pouring and weighing of powders and the transfer of powder from one vessel to another, become slow and cumbersome to perform and meticulous clean up of the containment facility is necessary after each experiment.
A variety of systems have been used to generate atmospheres containing toxic aerosols. Systems typically incorporate two basic mechanisms. First, a metering device is used to control the flow rate of the powder to be tested. Second, a pneumatic device, such as a high velocity/sonic air jet or a fluidized column of beads, is used to disperse the powder into an air stream which may then, for example, be administered to one or more test subjects in an inhalation toxicology study. Typical metering devices include helical screw feeders, rotary gear feeders, split tube feeders, rotating turntable feeders, and the like. While such devices may be capable of metering a freely flowing powder when the output is averaged over a long period, short duration feed rates vary considerably, and they are unsatisfactory for applications requiring flow rates that are substantially steady and controllable over relatively short periods. In addition, with the exception of the rotary turntable feeder, such devices require a minimum of several grams of powder to operate which rules them out for use where only minute quantities are to be tested. While the rotary turntable feeder is capable of operating with quantities of powder in the milligram range, an operator must ensure that the powder is uniformly spread out over the turntable in order for the device to perform satisfactorily.
These and other problems are addressed, at least in part, by embodiments of the present invention which provide for the delivery of a powder aerosol at a stable and controllable rate suitable for short duration applications. While it is anticipated that embodiments of the present invention will find use in a variety of applications where controlled delivery of a powder aerosol is needed without departing from the spirit and scope of the invention, the embodiments illustrated below may be used in performing short duration inhalation toxicology studies of extremely toxic aerosol powers.